Switch to C++11 style [[noreturn]].

Also remove old comment about calls to Abort getting merged. We have few
LOG(FATAL)s so merging is less of an issue. Also, recompiling with O0 and
similar will avoid the optimization and we expect that for GDB debugging.

Change-Id: I840dc6f4b8511294be0e117c634ec15b745e7be5